Yeh your right, few threads back i asked a similr think for min to add a way of counting verts but as far as i know, hes not got around to it yet (although i use my own model format / importer).
Anwyays, heres the thing. Min could add these but i think it would then only be possible to make this a pre-loading process and not a realtime process due to optimisation. But without tesitng then we wouldnt know. So Basically, it would be a case of load the model, go threw all verts and calculate the tangents, store them inside an array and then you have enough enough (tangents, normal map data, and textures) to send to you shader programs for drawing.
Its when animations are then involved, i suspect you would ahve to re-calculating tangents and normals and depending on how optimised your formula for calculatng them is and how complex your model is and how fast the computer and graphics card is, depends on the result.